The replicative pathway of measles virus will be anlyzed by means of genetic studies with temperature sensitive mutants and biochemical studies of RNA produced by virus in infected cells during complete and temperature aborted infection. Host cell contributions to this process will be sought by comparing blockage points in mutant and irradiation or drug damaged cell cultures. Relation to the whole human host will be investigated by studying measles immunity patterns in populations with and without reexposure to the virus.